1. Field of the invention
The present invention relates to a method of reproducing a chrominance signal in converting a low-range-converted chrominance signal from a recording medium into a carrier chrominance signal having a prescribed carrier frequency or demodulating the low-range-converted chrominance signal into a color difference signal so that the low-range-converted chrominance signal can be reproduced on a television receiver or the like. The method of the invention is suitable for use in signal reproducing systems such as video disc players or video tape recorders (VTR) for reproducing video signals from recording mediums such as video discs in which a chrominance signal in a video signal is recorded with a high packing density at a low-range conversion carrier frequency or video tapes in which a carrier chrominance signal is converted in a low frequency range and recorded
2. Description of the Prior Art
Most conventional video signal reproducing apparatus for home use reproduce video signals from recording mediums in which a carrier chrominance signal is converted into a low frequency range and recorded by frequency multiplexing below a luminance signal band which is frequency-modulated. Such a video signal recording system is referred to as a so-called "color-under" system.
FIG. 1 of the accompanying drawings shows in block form an arrangement for carrying out a conventional method of reproducing a chrominance signal.
A low-range-converted chrominance signal recorded in the color-under system is applied as a signal C.sub.L through a terminal 1, and processed by a balanced modulator 2, a comb filter 3, and a bandpass filter 4 into a carrier chrominance signal Csc having a carrier frequency equal to a chrominance subcarrier frequency fsc of a television signal, the carrier chrominance signal Csc being available through a terminal 5. Where an NTSC television signal is to be reproduced by a VHS-VTR, for example, a carrier to be applied to the balanced modulator 2 has a frequency fsc+fc=4.2 MHz and is produced via a bandpass filter 7 by a balanced modulator 6 which adds a low-range conversion carrier (having a frequency fc=629 KHz) and a carrier (having a frequency fsc=3.58 MHz).
FIGS. 2(a)-2(c) are a set of spectrum diagrams showing the manner in which frequency conversion is carried out. FIG. 2(a) shows a spectrum of the low-range-converted carrier chrominance signal C.sub.L which has a frequency band of .+-.500 KHz with a central frequency of 629 KHz. FIG. 2(b) shows a spectrum of an output signal of the balanced modulator 2, including the difference between and the sum of frequency components of the conversion carrier fsc+fc (4.2 MHz) and the signal C.sub.L, the difference being indicated by Csc and the sum by C'sc. The signal Csc which is required is picked up by the bandpass filter 4 having characteristics shown in FIG. 2(c). Since the signal C'sc which is an upper side band causes disturbance, the bandpass filter 4 is required to have sharp cutoff characteristics in a high frequency range. The bandpass filter 4 is also required to have cutoff characteristics for sufficiently attenuating the component fsc+fc since a carrier leak may be produced dependent on the characteristics of the balanced modulator 2.
Therefore, the conventional arrangement has required two balanced modulators 2, 6 and two bandpass filters 4, 7 for converting the low-range-converted chrominance signal into the carrier frequency, resuling in an increased circuit area and an increased number of circuit components needed. In addition, inasmuch as the bandpass filter 4 should have sharp cutoff characteristics, the band of a reproduced chrominance signal is narrowed to cause iamge degradation such as a color blur. A large group delay distortion is produced within the pass band of the filter, also causing image degradation. With the conventional chrominance reproducing method shown in FIG. 1, the bandpass filter 4 used in conversion into the carrier chrominance signal has been required to have high performance and has been one of major causes of chrominance signal degradation.
When it is necessary to issue a reproduced chrominance signal in a base band, the conversion carrier fsc+fc may be selected to be fc and the bandpass filter 4 may be replaced with a low-pass filter for passing the base band. However, the low-pass filter should have sharp cutoff characteristics for removing an upper side band component of the carrier fc which appears in addition to the chrominance signal in the base band, and such a low-pass filter is responsible for deteriorated images.
The comb filter 3 serves to remove crosstalk components from adjacent tracks in the VTR or the like. Where an NTSC television signal is to be reproduced on a VHS-VTR, the low-range conversion carrier in each track is advanced or delayed 90.degree. in phase in each horizontal period so that crosstalk components in the signal having passed through the balanced modulator will be brought into phase with each other in one horizontal period (this phase shifting will be referred to as an "NTSC-PS process"). On Beta-VTR or 8 mm-VTR, the low-range-converted chrominance signal is inverted in phase in each horizontal period to cancel crosstalk components (this phase inversion will be referred to as an "NTSC-PI process"). Where a PAL television signal is to be reproduced, the low-range conversion carrier in one of adjacent tracks is advanced or delayed 90.degree. in phase so that crosstalk components will be brought into phase with each other in every two horizontal periods (this phase shifting will be referred to as a "PAL-PS process").
FIG. 3 is a frequency spectrum of a signal prior to passing through the comb filter 3 at the time the carrier chrominance signal of an NTSC television signal is to be reproduced. Since crosstalk components are shifted by f.sub.H /2 with respect to a carrier chrominance signal spectrum indicated by the solid-line arrows, and the frequency fsc is 455/2 f.sub.H, the crosstalk components are removed by the characteristics, indicated by the solid line, of a comb filter composed of a 1-H delay line and a subtractor.
The 1-H delay line in the prior comb filter 3 comprises a glass delay line. Because the delay needed is large, i.e., one horizontal period (or two horizontal periods for PAL), and the delay should accurately be 1 H or 2 H, and also because the delay line is required to have a band of the carrier chcominance signal, the glass delay line is expensive and has an increased area. Inasmuch as the delay achieved by the glass delay line is constant, the comb filter will be ineffective if the spectrum of the carrier chrominance signal is shifted due to a deviation of the carrier frequency fsc of the carrier chrominance signal or jitter of the reproduced chrominance signal.
The carrier fsc supplied to the balanced modulator 6 is generated by a reference oscillator 8 of good characteristics employing a piezoelectric crystal. The low-range conversion carrier fc is produced by extracting a burst of the carrier chrominance signal Csc after being converted in frequency through a burst gate 9, comparing the phases of the extracted burst and the carrier fsc from the reference oscillator 8 with a phase comparator 10, applying the result of comparison through a low-pass filter to a voltage-controlled oscillator 12, and frequency-dividing an output signal from the voltage-controlled oscillator 12 and effecting the PS and PI processes on the output signal in a signal generating circuit 13. The voltage-controlled oscillator 11 is normally operated at a frequency which is four times the low-range conversion carrier. Generally, m is selected as an integer, a frequency mfc from the voltage-controlled oscillator 12 is frequency-divided into a frequency fc, and the signal is subjected to the PS and PI processes in each horizontal period using a horizontal synchronizing signal f.sub.H from a terminal 14 as a reference. A signal PG from a terminal 15 is a signal for discriminating tracks and is used as a switching signal for delaying or advancing the low-range conversion carrier fc in phase in each horizontal period when the NTSC-PS process, for example, is to be performed. As described above, a phase-locked loop is constituted by producing the low-range conversion carrier fc, and the burst of the carrier chrominance signal Csc is in synchronism with the carrier fsc issued from the reference oscillator 8.
There has been a demand in recent years for digital signal processing in the foregoing chrominance signal reproducing method and for making the parts such as the comb filter 3 of a semiconductor to render the circuit arrangement small in size and achieve a low power requirement. However, it has been difficult to produce a digital circuit arrangement since the frequencies used are high, and such a digital arrangement would be costly to manufacture. Another reason for the difficulty in achieving digital signal processing is that the balanced modulators 2, 6 and the phase comparator 10 operate on analog signal processing.